Certain phenomena need to be identified essentially by their spatial distribution such as the nature of the flow, for example laminar on a part of the wing and turbulent on another part. The action on the actuators then makes it possible to render the flow as laminar as possible, thereby reducing the drag. A characterization device makes it possible to detect a boundary layer detachment, and therefore to actuate devices for countering this detachment. A characterization device also makes it possible to identify and to locate the recompression shock wave phenomena at the supersonic-subsonic transition.
Other phenomena may be identified essentially by their dynamic characteristics at a given point such as oscillations in aerodynamic pressure, well known in the literature by the name “buffeting”, or phenomena of vibratory coupling between the aerodynamic forces and the elastic mechanical forces exerted on a wall that is not infinitely rigid. These coupling phenomena are charted by means of a characterization device used in combination with mechanical-load sensors, typically inertial or strain gauge sensors. These coupling phenomena are well known in the literature by the name “flutter”. These dynamic phenomena are uncomfortable and sometimes dangerous, and risk diverging to the extent that the structure of the aircraft breaks.
The invention finds another utility in the characterization of the aerodynamic stream along a boat sail making it possible to propel it. The sails are trimmed so as to exhibit a hollow profile. It is known to adjust the profile of the sails notably by modifying the hollow by acting on the tension of halyards making it possible to hoist the sails home. Moreover, the nature of the air flow along the sails may be visualized by means of dog-vanes arranged on the sail either on the intrados or on the extrados. These are strands, for example of wool, intended to follow the flow. In laminar flow, these strands cling to the sail and in turbulent flow these strands ripple. The crew of the boat monitor these dog-vanes and adjust the profile of the sails to obtain the most laminar flow possible in order to limit the drag. The function of the dog-vanes is the same, in a simplified manner, as that of the invention: to ascertain the nature of the flow at places of interest, so as to act on this flow.
In the field of sports sailing, this type of adjustment has remained manual in that a man watches the dog-vanes and corrects at a limited tempo with respect to the local aerological variations related to gusts and to waves for example.
In aeronautics, shear sensors well known in the literature by the name “shear-stress sensors” have been used to measure the speed of an aerodynamic stream along a wall. These sensors use for example the thermal transfer between a heating element situated on the wall and the aerodynamic stream. The temperature of this heating element is servo controlled and the power dissipated by the heating element to obtain the setpoint temperature is representative of the speed of the flow of the aerodynamic stream along the wall. Indeed, the more significant the speed of the aerodynamic stream, the more the wall is cooled by the aerodynamic stream and therefore the more significant the heating power required in order to reach the setpoint temperature.
The heating element must be thermally decoupled as well as possible from the material of the wall itself, so as to maintain the bandwidth of the measurement. In order for there to be heat energy transfer, the element is heated to a temperature generally regulated to a stable value greater than that of the aerodynamic stream. The heating power is generally provided through the Joule effect. The temperature regulation is generally ensured by measuring the temperature, either via a thermosensitive element separate from the heating element, or via the resistance of the heating element itself, which varies as a function of temperature. A regulating device acts either on the supply voltage of the heating element, or on the duty ratio of a fixed-voltage modulation. Thus, the thermal power transferred from the aerodynamic stream to the element is directly proportional to the electrical power provided to the heating element. The thermal decoupling of the heating element with respect to the wall is significant so that the cooling of the heating element occurs predominantly through the aerodynamic stream and to a lesser degree through the wall itself.
This principle is found in hot-wire sensors, in hot-film sensors and in sensors produced on the basis of a hot-diaphragm micro-machined structure.
This type of node is incompatible with use on commercial aircraft during operation. Indeed, hot-wire devices are not nearly robust enough to be arranged on a wing in mass production. Moreover they are very sensitive to particulate depositions, particularly water. Finally, their integration into the skin of an aircraft wing while maintaining industrial simplicity of manufacture is not solved.
Hot-film devices exhibit the same robustness drawbacks. Moreover, the presence of electrical signals outside the aircraft poses the unsolved problem of electrical and radio-electric susceptibility, notably in relation to lightning.
Hot-diaphragm micro-machined devices do not impose any protuberance. Nonetheless, the radio-electric susceptibility and the cost of integration of multiple sensors in the skin of the aircraft are not solved.
For the record, mention will be made of the existence, for boundary layer characterization applications, of infrared camera-based laminar transition detectors, which are usable only within the context of in-flight trials. Indeed, heating is ensured by the sun or by a powerful source distributed in the skin. These detectors also are not robust enough for use in bulk. Moreover, these detectors have insufficient decoupling with the environmental temperature variations.
All these devices measure directly and without distinction the transmitted thermal power variation, be it related to a variation in aerodynamic stream, or to a variation in the temperature of this aerodynamic stream, or even directly of the sensitive element by solar radiation for example, or by ambient electromagnetic radiation.
There also exist shear sensors which do not rely on variations in thermal transfer, but on pressure variations. This principle is found in traditional pressure sensors, with communication with the exterior through a hole in the skin of the aircraft, and surface pressure sensors, generally piezoelectric, distributed in the form of a film containing a multitude of sensors.
Traditional pressure sensors exhibit the major drawback of requiring a hole for communication with the exterior. This hole is subject to obstruction and therefore application during operations is compromised. The complexity of integrating a large number of pressure sensors under the skin of the aircraft is also a major drawback.
Arrays of pressure sensors distributed at the surface have the same drawbacks as hot films.
All these devices measure directly and without distinction all the pressure variations, be they related to a variation due to the local intrinsic characteristics of the stream in the boundary layer, in immediate proximity to the wall, or to acoustic noise of more distant origin, or even directly transmitted by the structure for accommodating the sensors, that is to say the wall.
All these devices therefore exhibit the drawback of insufficient decoupling with the environmental pressure variations.
These characterization devices make it possible, by means of local measurement of mean speed of the aerodynamic stream traveling along the wall, to characterize the type of flow but do not take account of the dynamic phenomena mentioned above.
There also exist numerous sensors for measuring speed or flowrate of a fluid traveling through a pipeline or along a wall. One finds sensors implementing principles described above such as hot-film sensors and hot-diaphragm micro-machined sensors.
There also exist optical fiber sensors of Bragg grating type, well known in the literature by the name “Fiber Bragg Grating”. The latter type of node has been known since 1978; the principle and embodiments thereof will be found in the following publication: Hill, K. O. (1978). “Photosensitivity in optical fiber waveguides: application to reflection fiber fabrication”. Appl. Phys. Lett. 32, 647.
Speed or flowrate optical sensors such as these have also been described and used to measure the speed of the aerodynamic stream, its temperature, or the flowrate of a pipeline for example. Such descriptions will be found, by way of example, in the documents: GB 2 389 902, DE 10 2006 04261, U.S. Pat. No. 6,431,010, WO 2004/094961, JP 2005/172713.
Sensors for measuring speed or flowrate, based on optical principles, do not carry out any characterization of the nature of the stream. More generally, they deal only with the mean speed or with the mean flowrate and would not make it possible to identify dynamic phenomena.